An APD is typically composed of a body of .pi.-type conductivity semiconductor material having a region of n-type conductivity extending a distance into the body from a surface thereof with a p-type conductivity region extending a further distance into the body from the n-type region, creating a p-n junction therebetween. A p.sup.+ -type conductivity contacting region extends a distance into the body from an opposite surface thereof. Electrical contact is made to the contacting region and the n-type region. In the operation of this APD, an applied reverse bias voltage produces an electric field within the APD whose profile depends upon the impurity concentration in the different regions and which forms a depletion region reaching into the .pi.-type region. Light incident on the surface containing the contacting region enters the photodiode and is absorbed primarily in the .pi.- or p-type regions, generating electron-hole pairs. The electrons are accelerated by the electric field until they attain sufficient energy for multiplication.
Webb, in U.S. Pat. No. 4,129,878, has disclosed an APD array in which the contacting region is segmented into a plurality of regions, each having a separate electrical contact. Such a device has the disadvantages that the electrical isolation between adjacent elements is poor and that a portion of the light entering the device will be masked by the electrical contacts to the individual elements. Segmentation on the p-n junction side of the device has not been satisfactory for several reasons. In an avalanche p-n junction the diameter of the n-type region is greater than that of the p-type region to prevent edge breakdown, thereby increasing the dead space between elements of an array. A large spacing is also required between the elements to prevent voltage breakdown between the elements. It would be desirable to have an APD array which has a plurality of closely spaced p-n junctions with good electrical isolation between the individual elements, a reasonably uniform gain and minimized dead space between the elements.